There are various systems for detecting whether or not an object is present and changing a control in response to a detection of the presence of the object. Taking rain drops as an example of the object, a windshield wiper control device of a windshield of an automobile has to change a control flexibly in response to a weather change leading to the start of rainfall. Development of a rain sensor for detecting whether or not it is raining is an important issue to enhance the convenience of a windshield wiper control device. Hereinafter, a conventional object-sensing device will be described by taking as an example a conventional rain sensor for detecting rain drops on a windshield of an automobile as an object.
In the case of a windshield wiper operated manually, which commonly is used, drivers have to recognize that it started to rain and manually switch a windshield wiper from off to on in view of the running state of the automobile and a change in the amount of rain drops attached on the windshield in order to ensure a necessary view through the windshield for driving the automobile. In order to reduce the trouble of this manual switching operation of the windshield wiper, a rain sensor is provided to detect the presence of an object such as rain drops on a sensing surface of the windshield of an automobile so as to determine whether or not the windshield needs wiping.
As a conventional rain sensor, rain sensors employing reflected light to detect rain drops are known. FIG. 23 shows a view simply illustrating the principle of detecting rain drops by a conventional reflected light sensing type rain sensor. In FIG. 23, reference numeral 1000 denotes a windshield of an automobile. For convenience, the upper space of the windshield 1000 is inside of the automobile, that is, the space on the side of a driver, and the lower space is outside of the automobile. Reference numeral 1010 denotes a light source, 1020 denotes a prism, 1030 denotes a prism for guiding reflected light out from the inner portion of the windshield, 1040 denotes a lens, 1050 denotes a PD (Photo Detector) as a light-receiving element, and 1110 denotes a sensing surface. Reference numeral 1120 denotes a rain drop on the sensing surface. The light source 1010 irradiates light flux spread widely enough to cover the entire sensing surface, and among the light flux, 1130 denotes a locus of the light incident to a portion in which the rain drop is attached. Reference numeral 1140 denotes a locus of light other than the light 1130, which is incident to a portion of the sensing surface on which the rain drop is not attached.
In the reflected light sensing type rain sensor, it is important to adjust the angle at which each component is provided and the material thereof (in particular, the refractive index of the material). Describing the basic principle of detecting rain drops, when a rain drop is on a sensing surface, incident light escapes to the outside because a total reflection condition is not satisfied in the external surface of the windshield 1000. On the other hand, when a rain drop is not attached on a sensing surface, the total reflection condition is satisfied in the external surface of the windshield 1000 and incident light is reflected totally. The reflected light sensing type rain sensor detects a difference in the intensity of the reflected light in these two cases.
Therefore, for the light source 1010 and the prism 1020, the angles and the materials are selected so as to satisfy the incident condition under which irradiated light is incident to the inner portion of the windshield 1000. In addition, an angle that allows total reflection in the sensing surface on the external surface of the windshield 1000 is selected. Furthermore, an angle at which light is incident with respect to the sensing surface 1110 is selected so that the total reflection condition on the sensing surface is switched so as to be satisfied or not satisfied in response to a change of the refractive index due to rain drops.
For the prism 1030, the material and the angle are selected so as to satisfy an outgoing condition under which reflected light can exit from the windshield 1000, that is, so as not to satisfy the total reflection condition. For the lens 1040 and the light-receiving element 1050, the angles and the distance are adjusted such that light incident to the lens 1040 is focused on the sensor portion of the light-receiving element 1050.
These components 1010 to 1050 can be provided on places other than the windshield 1000, for example, on the hood or the roof. However, since the state of the windshield 1000 is to be detected, it is preferable to provide these components in a portion of the windshield 1000. It is also preferable to provide these components so as not to hinder the view of a driver. For example, it is preferable to provide these components in a portion of the windshield in which the view originally is shielded by a rearview mirror.
The operation of the above-described conventional reflected light sensing type rain sensor will be described below. The light flux irradiated from the light source 1010 is admitted to the internal portion of the windshield 1000 by the prism 1020 and is incident onto the entire surface of the sensing surface 1110. Now, it is assumed that a rain drop 1120 is attached on the sensing surface 1110. Among the light incident on the sensing surface 1110, the light 1130 that has been incident to the portion in which the rain drop 1120 is attached escapes to the outside because the total reflection condition is not satisfied because of the rain drop having a refractive index n of about 1.3 on the external surface of the windshield 1000, and this light is not detected by the light-receiving element 1050. On the other hand, among the light incident to the sensing surface 1110, the light 1140 that has been incident on a portion to which the rain drop is not attached is totally reflected because the total reflection condition is satisfied because of the air having a refractive index n of about 1 on the external surface of the windshield 1000. The totally reflected light is not totally reflected due to the presence of the prism 1030 on the surface of the windshield 1000 inside of the automobile and exits to the interior of the automobile. The light that has exited is focused on the photosensor portion on the light-receiving element 1050 by the lens 1040.
Thus, the amount of light detected by the light-receiving element 1050 decreases when the rain drop 1120 is present, and the larger the area of the sensing surface 1110 that is covered with the rain drop 1120 becomes, the less the amount of light detected becomes. This change of the light amount is detected to determine the presence of the rain drop on the sensing surface 1110. The above is the principle of the rain drop detection by the conventional reflected light sensing type rain sensor.
Each type of a rain sensor is configured so as to output a rain drop detection signal when a signal change as described above is detected. The rain drop detection signal from the rain sensor is input to a control portion of a windshield wiper, and the windshield wiper is controlled as desired in response to the input of the rain drop detection signal.
However, the conventional rain sensor has the following problems.
A first problem is that although the presence of an object can be detected with the conventional rain sensor, the type of the object and the state are not estimated. The main purpose of the rain sensor is to control the driving of the wiper that wipes the windshield 1000, if necessary, and there are some cases where the driving of the wiper has to be switched in accordance with the type and the state of an object attached on the windshield 1000. The conventional rain sensor cannot estimate the type of the object and the state thereof, so that it cannot but perform the same and uniform control.
The object attached on the windshield 1000 is not necessarily water drops due to rainfall. Various kinds of objects can be thought of, such as muddy water scattered during driving and droppings dropped by birds such as pigeons. The state of the object is varied, and even when water is attached, various states can be thought of, such as in the state of fogging on the windshield, or in the state of frozen ice caused by temperatures dropping in winter. In the case of muddy water, there are a state with a large water content and a dry state with a small water content. Also in the case of droppings of birds, there are soft droppings and dry droppings.
It is preferable to switch the driving of the wiper in accordance with the type and the state of the object attached on the windshield 1000. For example, when water is frozen, or when muddy water or droppings of birds are dry, wiping only with a wiper is not effective, and what is worse, the device or the windshield may be damaged. In this case, for example, it is preferable to drive the wiper while spraying cleaning liquid.
A second problem is concerned with the precision of object detection. In the conventional rain sensor, an absolute value of a signal level of the amount of light detected by the light-receiving element is analyzed, or signal levels detected at a constant time interval are stored, and temporal changes of the signal levels are analyzed so as to detect the presence of an object on the sensing surface. The temporal changes of the detected signal levels are slightly different, depending on the type of the object and the object state, but it is not easy to detect such a slight change precisely and estimate the type of the object and the object state.